Amplifier linearisation

ABSTRACT

The present invention relates to a lineariser for use with an amplifier and to a method of linearising an amplifier. The lineariser comprises an input to receive an input signal and an output for outputting an adjusted signal. A gain variations adjustment means are provided for adjusting amplitude dependent gain variations of the signal. A phase variations adjustment means are provided for adjusting amplitude dependent phase variations of the signal. Said gain and phase variations adjustment means are adapted to be individually adjustable elements on the signal path between said input and output. A variable gain amplifier is provided on the signal path between the gain variations adjustment means and phase variations adjustment means. The adjusted signal is then input to said amplifier.

FIELD OF THE INVENTION

[0001] The present invention relates to linerisation of an amplifier,and in particular, but not exclusively, to a lineariser for use with apower amplifier for amplifying radio frequency (RF) signals. Alineariser implemented in accordance with the invention may be used inelements of a communication system.

BACKGROUND OF THE INVENTION

[0002] A communication system comprises signalling points or nodes, suchas user terminals, different exchanges, routers, switches, links,stations and so on. The communication also comprises appropriatecommunication media between the signalling points. Different signallingpoints may be situated within an element of the communication system,wherein the communication may occur with the element. The communicationmedia may comprise, for example, a wired interface, a radio frequencyinterface or an optical interface. The communication may be carried byanalogue or digital signals or a combination of these, such asdigitally-modulated analogue signals.

[0003] Signal amplification is required in various communicationapplications. For example radio frequency signals transmitted betweensignalling points in a communication system employing radio transmissionmay need to be amplified during some stage of the transmission and/orreception. The signalling points may be, for example, a transmittingstation and a receiving station or an intermediate node of thecommunication system. The amplification of the signals is required forexample since the amplitude of a signal tends to be attenuated duringtransmission between signalling points, thereby decreasing the qualityof the transmission. Also, noise becomes typically added to the signalfrom other sources as well as from the transmitting and receiving andthe possible intermediate apparatus itself. A communication system isthus typically provided with amplifying means to compensate for theattenuation. Amplification of the signal may also be used for increasingthe signal-to-noise ratio of the signal.

[0004] An input signal may be amplified by means of a power amplifier.The power amplifier may have at its input a modulated radio signal. Theinput radio signal may be a modulated digital signal. The poweramplifier, when operated in a non-linear region, typically close tosaturation produces intermodulation distortion products at its output aswell as the desired carrier signals. The intermodulation distortionproducts are typically produced on either side of the desired carriersignals. Intermodulation distortion products are typically caused by thepower amplifier not acting as a linear amplifier, which occurs when thepower amplifier is operating close to saturation. The intermodulationdistortion produces frequencies at multiples of the carrier frequency ofthe desired signals. These frequencies tend to be lower in signalstrength than the desired carriers. These intermodulation distortionproducts increase the spectral space occupied by the signal and aretherefore undesirable. One way to reduce the intermodulation distortionproducts is to operate the power amplifiers as linear amplifiers. Thepower amplifiers would be operated so that there is a substantiallylinear relationship between the input signal power and the output signalpower.

[0005] Amplifiers that are intended to cover a range of frequenciesshould provide as linear performance as possible across the designatedfrequency band. However, an amplifier may introduce distortions. Thedistortion may be linear or non-linear. The linear distortions relate tobandwidth limitations. The non-linear distortions have two componentscalled AM-AM (amplitude modulation-amplitude modulation) or AM-PM(amplitude modulation-phase modulation) distortion. In the non-linearcase amplitude variations in the input signal may cause undesirableamplitude and/or phase variations in the output signal. In addition, thedistortions may cause mixing between the different frequency componentspresent in the signal. The term ‘AM-AM’ refers to amplitude dependentamplitude (gain) variations and the term ‘AM-PM’ refers to amplitudedependent phase variations.

[0006] A prior art solution for the linearity problem exploits the factthat the non-linearity increases with the output power level of theamplifier. Thus, if the input level is reduced, i.e. “backed-off”, theamplifier is arranged to operate only within its more linear region.However, this approach may not be desirable in all applications as itfails to utilise the full range of available output voltage-swing. Thebacking-off may have an disadvantageous effect on power efficiency ofthe amplifier.

[0007] The linearity of the amplifying function can also be improved byprovision of a linearisation function designed to reduce the distortion.Linearisers have been developed which enable operation of an amplifierwith reduced intermodulation distortion at a point on the gain/inputpower or phase/input power curves after which the amplifier does not actas a linear amplifier. Such a lineariser is placed in the signal pathbefore the power amplifier and may therefore precondition the inputsignal before passing it to the power amplifier. In other words the gainof the lineariser increases as the power of the input signal increasesand increases in such a way as to substantially oppose the typical poweramplifier power characteristics. This is called gain expansion. Inaddition, the linearizer may change the phase of the input signal as thepower of the input signal increases. The change in the phase may be suchthat it opposes the typical power amplifier phase characteristics. Thisis called phase expansion.

[0008] A lineariser should be capable of producing enough of gain and/orphase expansions to mitigate the non linear amplitude and phase problemsassociated with power amplifiers in a region that is close tosaturation. The effect of the lineariser is to increase the effectiverange over which the power amplifier is linear but allows the poweramplifier to operate in its more efficient non-linear range. However,the inventor has found that the prior art linearisers may not be capableof providing an optimised independent amplitude and phase tuning forlinearity and efficiency of the prior art lineariser arrangements. Inaddition, the prior art linearisers are of substantially big size,thereby making the integration thereof with the amplifier difficult inapplications requiring substantially small sized components, such asmobile phones.

SUMMARY OF THE INVENTION

[0009] Embodiments of the present invention aim to address one orseveral of the above problems.

[0010] According to one aspect of the present invention, there isprovided a lineariser for use with an amplifier comprising: an input toreceive an input signal; an output for outputting adjusted signal tosaid amplifier; a signal path between the input and the output; a gainvariations adjustment means on the signal path for adjusting amplitudedependent gain variations of the signal; a phase variations adjustmentmeans on the signal path for adjusting amplitude dependent phasevariations of the signal, said gain and phase variations adjustmentmeans being individually adjustable elements; and a variable gainamplifier located on the signal path between the gain variationsadjustment means and phase variations adjustment means.

[0011] The arrangement is preferably such that the adjustment of theamplitude dependent gain variations does not produce any substantialamplitude dependent phase variations in the signal and/or vice versa.

[0012] The variable gain amplifier may be arranged for shiftingamplitude dependent phase and gain variation curves relative to eachother, and more particularly, the variable gain amplifier may beprovided for shifting the transfer characteristic curves of theamplitude dependent phase and gain variations relative to each other.

[0013] A second variable gain amplifier may be provided for adjustingthe power level of the signal to be input in said amplifier. A thirdvariable gain amplifier may be provided for adjusting the power level ofthe input signal.

[0014] The gain variations adjustment means may comprise a field effecttransistor (FET). The phase variations adjustment means may be providedwith a non-linear resistor element.

[0015] The lineariser may be implemented as a microwave monolithicintegrated circuit.

[0016] The signal may be a radio frequency signal transmitted between abase station and a mobile station of a cellular telecommunicationsystem.

[0017] According to another aspect of the present invention there isprovided a method of linearising an amplifier, comprising: inputting asignal in a lineariser; adjusting amplitude dependent gain variations ofthe signal independently from amplitude dependent phase variations ofthe signal; adjusting amplitude dependent phase variations of the signalindependently from the adjustment of the amplitude dependent gainvariations of the signal, the amplitude dependent gain variations andthe amplitude dependent phase variations being adjusted by separateadjustment means; adjusting the power level of the signal by a variablegain amplifier located on the signal path between the gain variationsadjustment means and phase variations adjustment means; and outputtingthe signal from the lineariser to be input in said amplifier.

[0018] The embodiments of the invention may improve the linearity of aradio frequency amplifier. The embodiments may also enable efficient useof the amplifying range of an amplifier. In addition, the embodimentsmay enable an improved adjustment for the linearisation. The embodimentsmay also improve the possibilities to integrate a lineariser with anamplifier in applications requiring substantially small and/orintegrated components.

BRIEF DESCRIPTION OF DRAWINGS

[0019] For better understanding of the present invention, reference willnow be made by way of example to the accompanying drawings in which:

[0020]FIG. 1 shows one embodiment of the present invention;

[0021]FIG. 2 shows an amplitude dependent gain variations block for usein the embodiments of the invention;

[0022]FIG. 3 shows an amplitude dependent phase variations block for usein the embodiments of the invention;

[0023]FIG. 4 illustrates gain and phase responses for the block shown inFIG. 2;

[0024]FIG. 5 illustrates gain and phase responses for the block shown inFIG. 3;

[0025]FIG. 6 illustrates the shifting between amplitude dependent gainvariations and amplitude dependent phase variations curves;

[0026]FIG. 7 is a flowchart illustrating the operation of one embodimentof the present invention; and

[0027]FIG. 8 shows a system wherein a lineariser in accordance with theinvention may be employed.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0028] Reference is made to FIG. 1 which shows a block diagram of alineariser i.e. predistorter 10 that has been implemented in accordancewith an embodiment of the present invention. More particularly, FIG. 1shows an analogue predistorter configuration that is suitable forintegration with an amplifier 5 so that they form a module. Thepredistorter is placed before the amplifier on the signal path, i.e.such that the signal output from the distorter element 10 is input tothe amplifier 5.

[0029] The predistorter 10 is provided with two separate adjustmentblocks 1 and 2. By means of this the analogue predistorter 10 is adaptedto enable independent or orthogonal adjustment of the AM-AM and AM-PMtransfer characteristics of the lineariser. By means of tuning the AM-AMand AM-PM characteristics of the lineariser it is possible to make thecascade of the lineariser and amplifier combination to behave in a morelinear manner.

[0030] More particularly, the predistorter 10 provides a possibility totune the AM-AM and AM-PM transfer characteristics independently relativeto each other. The independent adjustment can be accomplished such thatthe power level point (so called knee point) where the AM-AM correctionstarts and the power level point where the AM-PM correction starts canbe shifted relative to each other. This relative shifting is illustratedin more detail by the diagrams of FIGS. 4 to 6.

[0031] As shown in more detail by FIG. 2, the AM-AM adjustment block 1comprises of a FET 11 (Field Effect Transistor). The arrangement is suchthat the AM-AM block 1 is adapted to produce only AM-AM gain expansionwhile the AM-PM transfer characteristics of the lineariser are keptconstant. This is illustrated by the two diagrams of FIG. 4.

[0032] The FET 11 is preferably biased in a pinch-off region. In otherwords, the FET 11 is preferably operated close to i.e. in the vicinityof the pinch-off region so that the gain expansion characteristics ofthe FET 11 in this region can be utilised. The pinch-off region of theFET 11 is the region where the FET 11 is just switched on. The pinch-offregion is typically controlled by gate voltage.

[0033] The adjustment can be enabled by making it possible to tune thegate and drain bias voltages 12, 13 of the FET 11. This does notinfluence the AM-PM characteristics of the block 1 and these remainsubstantially flat (see AM-PM/Power curve of FIG. 4). The tuning isaccomplished such that an AM-AM/Power curve as illustrated by FIG. 4 canbe obtained. That is, the AM-AM block is tuned such that the AM-AMresponse of the cascaded lineariser and amplifier is made as flat aspossible. In the usable tuning range of the gate and drain voltages thechange of the shape of the AM-PM curve is negligible. The tuning rangeof the voltages is preferably wide enough so that an improvement in thelinearity can be obtained.

[0034] In the preferred embodiment the gate voltage is the main tuningvoltage. The drain voltage may then be used for fine tuning. However, itshall be appreciated that the tuning is not restricted to use of onlyone voltage but both of the drain and gate voltages can be used if thisis deemed necessary.

[0035] The matching blocks 14, 15 are for matching the impedances of theFET 11 to the impedances of other blocks connected to the AM-AM block 1.Inductor L, resistor R and capacitor C of FIG. 2 form a feedback network16 for the FET 11. The values of the components L, R and C arepreferably chosen such that a correct shape of the AM-AM can be obtainedso that the FET 11 can be tuned with the voltages, that as flat AM-PM aspossible is obtained, and that the matching of the FET 11 can be madeeasier.

[0036] The AM-PM adjustment block 2 is adapted to provide only AM-PMcharacteristics correction, that is a correction of the amplitudedependent phase variation. The correction is provided without affectingthe amplitude dependent gain i.e. the AM-AM transfer characteristics ofthe lineariser (see FIG. 5).

[0037] As shown in more detail by FIG. 3, the AM-PM block 2 consists ofa forward biased diode 21 and a capacitor 22. Use of a non-linearresistor element (represented by the diode 21) with capacitors enablesprovision of an adjustment block that produces a substantially flatAM-AM response. The adjustment is adapted to be accomplished by tuningthe bias voltage of the diode 21. More particularly, the diode 21 isshown to be supplied with bias voltage from a bias voltage source 23.The adjustment is done by varying the bias voltage supplied by thesource 23. The bias voltage is used to tune the slope of the AM-PMtransfer characteristics of the block 2. This tuning is illustrated bythe right hand side diagram of FIG. 5. A choke 24 may be provided forpreventing the RF signal from entering the bias network 23.

[0038] The predistorter 10 may also be provided with a first variablegain amplifier (VGA1) 3. The VGA1 can be used for setting the inputpower level for the AM-PM block 2. The variable gain amplifier 3 may beprovided for shifting the AM-AM and AM-PM curves within the predistorter10 relative to each other. The diagram of FIG. 6 illustrates an exampleof the shifting of the AM-AM and AM-PM transfer characteristics relativeto each others by changing the gain of the first variable gain amplifier3.

[0039] The predistorter 10 may also be provided with a second variablegain amplifier (VGA2) 4. The second variable gain amplifier 4 can beused to set the power level of the input signal for the RF amplifier.The variable gain amplifier 4 may be provided for shifting the AM-AM andAM-PM curves of the predistorter 10 relative to the AM-AM and AM-PMcurves of the amplifier 5 so that an improved linearisation of theamplifier can be achieved. The variable gain amplifier 4 may enable amore optimal linearity improvement.

[0040] It shall be appreciated that the function of the variable gainamplifiers of FIG. 1 is not limited to amplification of signals. In someembodiment the VGAs may also be used for attenuation of the signals.

[0041] As shown by FIG. 7, in operation the tuning of the AM-AM transfercharacteristic may be accomplished by adjusting the gate and drainvoltages of the FET 11. The tuning the AM-PM transfer characteristic ispreferably accomplished first by adjusting the bias voltage supplied tothe diode 21. The input power level for the AM-PM block 2 may then beadjusted by changing the gain of a variable gain amplifier 3 in order toshift the AM-AM and AM-PM transfer characteristic curves and thus thepower points thereof relative to each others.

[0042]FIG. 1 shows also a third variable gain amplifier 8 (VGA3). Thisvariable gain amplifier may be used to adjust the power level of theinput signals. By the provision of the VGA3 it is possible to widen theoperational range of the lineariser 10, thereby enabling use thereof ina wide range of different applications. In other words, the variablegain amplifier 8 at the input of the lineariser 10 functions to set anappropriate correct input power level for the lineariser orpredistorter. By means of this the lineariser 10 can be fitted to avariety of systems which have different power levels.

[0043] The predistorter or lineariser 10 is believed to provide acompact, substantially cheap and easily adjustable lineariser for an RFamplifier. It is possible to avoid the shortcoming of the prior artlineariser in that the adjustments of the gain characteristicsinfluenced the phase characteristics and vice versa. The circuitryenables shifting of the AM-AM and AM-PM responses relative to eachother.

[0044] It shall be appreciated that the placing of the AM-AM adjustmentblock 1 and the AM-PM adjustment block 2 is interchangeable, and shouldnot have any effect to the operation of the predistorter 10.

[0045] The predistorter may also be implemented as a MicrowaveMonolithic Integrated Circuit (MMIC). This type of implementation mayease the integration of the predistorter into a Power Amplifier (PA).MMICs as such are known and are used within many modern circuits forexample in satellite and mobile telephony technologies.

[0046] Reference is now made to FIG. 8 illustrating a system in whichlinearisers in accordance with the invention may be employed. Theexemplifying system is a cellular mobile radio communication systemallowing a plurality of mobile stations MS1, MS2, MS3 to communicatewith a base (transceiver) station BTS in a common cell via respectivechannels CH1, CH2, CH3. The radio communication between a transmittingstation and a receiving station may be implemented in any appropriatemanner and may be based on any communication standard. Therefore theradio link as such will not be described in more detail herein. Examplesof cellular communication systems include, without being limited tothese, standards such as AMPS (American Mobile Phone System), DAMPS(Digital AMPS), GSM (Global System for Mobile communications), EDGE(enhanced data rate for GSM evolution), GPRS (General Packet RadioService), CDMA (Code Division Multiple Access), IS-95 or any of the3^(rd) generation (3G) communication systems, such as WCDMA (WidebandCDMA), UMTS (Universal Mobile Telecommunications System), or IMT-2000(International Mobile Telecommunications System 2000) and so on.

[0047] The mobile stations as well as the base station are provided withnecessary transceiver components (not shown in FIG. 8) so as to be ableto handle signals to be transmitted and received by respective antennae.These components are known to a skilled person and do not as such form apart of the invention, and are thus not described in more detail. It issufficient to note that the components of a communication systemtypically include one or several power amplifiers.

[0048] The power amplifiers of the base station BTS or the mobilestations of FIG. 8 may be provided with linearisers as discussed above.However, it should be appreciated that the embodiments of the presentinvention are applicable to any other suitable type of equipmentemploying amplifiers.

[0049] It is also noted herein that while the above describesexemplifying embodiments of the invention, there are several variationsand modifications which may be made to the disclosed solution withoutdeparting from the scope of the present invention as defined in theappended claims.

1. A lineariser for use with an amplifier comprising: an input toreceive an input signal; an output for outputting adjusted signal tosaid amplifier; a signal path between the input and the output; a gainvariations adjustment means on the signal path for adjusting amplitudedependent gain variations of the signal; a phase variations adjustmentmeans on the signal path for adjusting amplitude dependent phasevariations of the signal, said gain and phase variations adjustmentmeans being individually adjustable elements; and a variable gainamplifier located on the signal path between the gain variationsadjustment means and phase variations adjustment means.
 2. A lineariseras claimed in claim 1, the arrangement being such that the adjustment ofthe amplitude dependent gain variations does not produce any substantialamplitude dependent phase variations in the signal.
 3. A lineariser asclaimed in claim 1 or 2, the arrangement being such that the adjustmentof the amplitude dependent phase variations does not produce anysubstantial amplitude dependent gain variations in the signal.
 4. Alineariser as claimed in any preceding claim, wherein said variable gainamplifier is for shifting the transfer characteristic curves of theamplitude dependent phase and gain variations relative to each other. 5.A lineariser as claimed in claim 4, wherein the turning point of theamplitude dependent phase variations curve is adapted to be shifted bythe variable gain amplifier.
 6. A lineariser as claimed in any precedingclaim, wherein a second variable gain amplifier is provided foradjusting the power level of the signal to be input in said amplifier.7. A lineariser as claimed in any of preceding claim, wherein a thirdvariable gain amplifier is provided for adjusting the power level of theinput signal.
 8. A lineariser as claimed in any preceding claim, whereinthe gain variations adjustment means comprises a field effect transistor(FET).
 9. A lineariser as claimed in claim 8, wherein the amplitudedependent gain characteristics of the signal are adjustable by adjustingthe gate voltage of the field effect transistor.
 10. A lineariser asclaimed in claim 8 or 9, wherein the amplitude dependent gaincharacteristics of the signal are adjustable by adjusting the drainvoltage of the field effect transistor.
 11. A lineariser as claimed inany of claims 8 to 10, wherein biasing means are provided to provide thefield effect transistor with a biasing voltage.
 12. A lineariser asclaimed in of claim 11, wherein the field effect transistor is biased ina pinch-off region.
 13. A lineariser as claimed in claim 11 or 12,wherein amplitude dependent gain variations of the signal are adapted tobe adjusted by means of adjusting biasing voltage of the field effecttransistor.
 14. A lineariser as claimed in any of claims 11 to 13,wherein the biasing means comprise a first biasing means for biasing thegate voltage and a second biasing means for biasing the drain voltage ofthe field effect transistor.
 15. A lineariser as claimed in anypreceding claim, wherein the phase variations adjustment means isprovided with a non-linear resistor element.
 16. A lineariser as claimedin claim 15, wherein the phase variations adjustment means comprises abiasing voltage source for provision of a biasing voltage for thenon-linear resistor element.
 17. A lineariser as claimed in claim 15 or16, wherein the non-linear resistor element comprises a forward biaseddiode.
 18. A lineariser as claimed in claim 16 or 17, wherein theamplitude dependent phase variations of the signal are adapted to beadjusted by adjusting the biasing voltage of the non-linear resistorelement.
 19. A lineariser as claimed in any preceding claim, wherein thephase variations adjustment means is provided with capacitor means. 20.A lineariser as claimed in any preceding claim being implemented as amicrowave monolithic integrated circuit.
 21. A lineariser as claimed inany preceding claim, wherein said input signal is a radio frequencysignal.
 22. A lineariser as claimed in claim 21, wherein said radiofrequency signal is a signal to be transmitted between a base stationand a mobile station of a cellular telecommunication system.
 23. Alineariser as claimed in any preceding claim, wherein said at leastvariable gain amplifier is adapted to amplify signals.
 24. A lineariseras claimed in any preceding claim, wherein said at least variable gainamplifier is adapted to attenuate signals.
 25. A lineariser as claimedin any preceding claim in combination with an amplifier. 26.Communications apparatus comprising a lineariser as claimed in any ofclaims 1 to
 24. 27. A communications apparatus as claimed in claim 26,wherein said communications apparatus is used for communication in acellular communications system.
 28. A communications apparatus asclaimed in claim 27, wherein said apparatus is a mobile station.
 29. Acommunications apparatus as claimed in claim 27, wherein said apparatusis a base station.
 30. An integrated circuit comprising a lineariser asclaimed in any of claims 1 to
 25. 31. An integrated circuit as claimedin claim 30, wherein said integrated circuit is a microwave monolithicintegrated circuit.
 32. A method of linearising an amplifier,comprising: inputting a signal in a lineariser; adjusting amplitudedependent gain variations of the signal independently from amplitudedependent phase variations of the signal; adjusting amplitude dependentphase variations of the signal independently from the adjustment of theamplitude dependent gain variations of the signal, the amplitudedependent gain variations and the amplitude dependent phase variationsbeing adjusted by separate adjustment means; adjusting the power levelof the signal by a variable gain amplifier located on the signal pathbetween the gain variations adjustment means and phase variationsadjustment means; and outputting the signal from the lineariser to beinput in said amplifier.
 33. A method as claimed in claim 32, comprisingthe step of adjusting the relative position between knee point of theamplitude dependent phase variations and the knee point of the amplitudedependent gain variations.
 34. A method as claimed in claim 33, whereinthe knee point of the amplitude dependent phase variations is adjusted.35. A method as claimed in claim 33 or 34, wherein the relativepositioning of the knee points is adjusted by said adjustment of thepower level of the signal.
 36. A method as claimed in any of claims 32to 35, comprising adjustment of a gate voltage of a transistor providedfor the adjustment of the amplitude dependent gain variations.
 37. Amethod as claimed in any of claims 32 to 36, comprising adjustment of adrain voltage of a transistor provided for the adjustment of theamplitude dependent gain variations.
 38. A method as claimed in any ofclaims 32 to 37, comprising adjustment of a biasing voltage supplied toa non-linear resistor element provided for the adjustment of theamplitude dependent phase variations.